Carbide precipitation and grain boundary plane selection in overaged type 316 austenitic stainless steel

The misorientation and the grain boundary plane distribution in an overaged type 316 specimen have been measured, coupled with observations of M23C6 precipitation. There were many grain boundary planes on {1 1 1} in the random boundary population, notably {1 1 1} twist boundaries and 〈1 1 0〉 asymmetric tilt boundaries having one {1 1 1} plane. Precipitation was selective with respect to boundary plane type. Only coherent twins were immune to precipitation. For both random boundaries and incoherent Σ3s, boundaries which were not on {1 1 1} had more precipitation than did {1 1 1} twist boundaries. The enhanced proportion of {1 1 1} twist boundaries promoted plate-like carbide coarsening, which is associated with reduced cavity nucleation. A microstructure having a high proportion of {1 1 1} boundary planes, especially {1 1 1} twists, is therefore beneficial. It is suggested that the long age allowed a ‘fine tuning’ effect at interfaces whereby they approached energy minima.